I'd like to chime in with the exact same problem. I have a WD FreeAgent Desktop 500GB, it used to run at 480mbit/sec with Tiger, but now when I try booting from it in Leopard it runs at 12mbit/sec instead. I'd say this is Apple's fault and not Seagate's but I'll be darned if I can find a solution... restoring my disk image (67GB) is taking me > 24 hrs, it used to be done over night on the EXACT SAME HARDWARE... this is killing me! Please Seagate guys, talk to Apple, this bug is making everyone look bad!
For the record, I'm on a Macbook Pro Core Duo 2Ghz 15" with a WD FreeAgent Desktop 500GB, and this problem is most obvious when trying to boot from the external drive. System Profiler clearly shows it as being 12mbit/sec when I know this wasn't the case before.